The candidates of Acquisition Mode selection and Trigger Source selection will be defined by the CSA editor. The CSA editor will assign some the data streams after the data conditioning as candidates of trigger sources. For example, in a CSA there are 8 channels, if you only select ch1 and ch2 time streams as candidates of trigger source and then this CSA will only show ch1 and ch2 on the trigger source selection menu.
You may also define time streams other than native channels as trigger source candidate. For example, if in the CSA an RMS measurement is derived from ch1, this RMS time stream can be used as a trigger source.